Use device pixel ratio to calculate size of portal for max representation size (with API)

[LloydW93]

This supersedes PR #1030 by @nickygerritsen - resolving the conflicts and adding an API to control the feature.

[dsparacio]

@LloydW93 Other than Dave's comment I think it LGTM

[LloydW93]

Dave followed this up after the comment and pointed out that almost nowhere else today runs that check. I'm not saying I shouldn't fix it (I will), but it's a thing worth being aware of more generally.

[davemevans]

I did follow up privately with @LloydW93. I was under the impression we checked more widely for window but, on looking at the codebase, this only happens in a couple of places. I'm inclined to merge this regardless of my comment as it LGTM too, and possibly open an issue to capture comment.